Combustion with air as oxidizer has been generally applied in conventional boiler systems for thermal power generation. In such air combustion boilers, coal is in heavy use as fuel because of richness in recoverable reserves. However, in the coal-using boiler systems, an amount of carbon dioxide generated per unit heat value is greater than that in boiler systems using other fossil fuel such as natural gas or petroleum. Thus, research and development on techniques for carbon dioxide capture and storage (CCS) has been actively advanced from a viewpoint of global warming countermeasures.
Thus, there have been proposed oxyfuel combustion boilers capable of enhancing a concentration of carbon dioxide by oxyfuel combustion to effectively capture the carbon dioxide (see, for example, Patent Literatures 1 and 2).
Patent Literatures 1 and 2 disclose techniques for enhancing a concentration of carbon dioxide by an oxyfuel combustion boiler using coal as fuel, air being separated by an air separation unit (ASU) into oxygen and nitrogen-prevailing gas (hereinafter, referred to as nitrogen gas), the resultant oxygen being supplied to a boiler furnace. In Patent Literature 2, recirculated flue gas extracted from the flue gas downstream of the boiler furnace is partly supplied as primary recirculated flue gas to a pulverizer to dry and pulverize coal, a fluid mixture of a resultant pulverized fuel with the primary recirculated flue gas being supplied to a burner of the boiler furnace. A remainder of the recirculated flue gas is supplied as secondary recirculated flue gas to the boiler furnace (e.g., a wind box thereof), the secondary recirculated flue gas being adjusted to adjust a concentration of oxygen supplied from the ASU to thereby stabilize the oxyfuel combustion of the pulverized fuel. As a result, the gas supplied to the boiler furnace for combustion of the pulverized fuel is mainly constituted by oxygen, carbon dioxide and water vapor, and combustion flue gas has a suppressed amount of nitrogen and is substantially constituted by carbon dioxide, so that a concentration of carbon dioxide in the flue gas can be substantially enhanced in comparison with that in conventional boiler systems where the flue gas has a great amount of nitrogen by the air combustion.